The present invention relates to a self-illuminated liquid crystal device and particularly to such a device in which the luminating means is self-powered.
Recently liquid crystal devices have come into use as displays for electronic watches, clocks and the like. Liquid crystal devices do not emit light, but rely on outside illumination transmitted through or reflected off of the device. Thus, liquid crystal devices are more legible in moderate to bright ambient illumination and are not very visible in dim ambient lighting or complete darkness.
In order to provide suitable illumination for a liquid crystal device in dim ambient lighting or darkness, a small light bulb has been mounted behind the device. To conserve power the light bulb is usually connected through a switch to a source of current, generally the battery that also operates the liquid crystal device. Thus, when additional illumination of the liquid crystal device is required it can be achieved by closing the switch. Although this does provide the desired illumination of the device when required, the bulb does use some of the power which would otherwise be used to operate the liquid crystal device. Also, it requires the operation of the switch which may not always be easily done, especially when the liquid crystal device is used as a watch display.
Another type of illumination which has been used in a liquid crystal device is shown and described in U.S. Pat. No. 3,722,206 to J. M. Bergey issued Mar. 27, 1973 entitled, "Self-Illuminated Liquid Crystal Time Piece". This type of illumination includes an enclosed glass tube coated on its inner surface with a cathodoluminescent phospher and filled with a radioactive gas, such as tritium. The radioactive gas emits low energy beta particles which strike the phosphor and thereby induce the emission of visible radiation. This radiation illuminates the liquid crystal device. This type of illumination has the advantage over a bulb in that it is self-powered and requires no switch. However, at present, it can only be made in small sizes and it provides only a low level of illumination, about 0.1 foot-lambert. Thus, several of these tubes, two or three, have been used across a liquid crystal display device to illuminate the entire display. However, the overall light from the tubes is not uniform because of non-luminous regions between the tubes so that the display device is not uniformly illuminated.